Paper TR+SE-WeM3
Tribological Surface Chemistry of Model Lubricant Additives Measured in Ultrahigh Vacuum

Wednesday, October 31, 2012, 8:40 am, Room 19

Additives are generally added to lubricants, which react with the surfaces to form a boundary lubricating film that can lower friction and/or prevent wear. At the high interfacial temperatures that occur under so-called extreme-pressure conditions, the surface reaction and film growth kinetics are dominated by thermal processes. In contrast, it is postulated that, under mild conditions, where the surface temperature rise is low, a surface film can be formed by a shear-induced, surface-to-bulk transport mechanism. This effect is investigated by studying the tribologically induced surface reactions of model sulfur- and boron-containing additives on copper surfaces in ultrahigh vacuum (UHV) where the background pressure is ~1×10^-10 Torr. The nature of the initial surface species that are formed by exposure of the copper surface to the model lubricant additive is investigated using a range of surface analytical techniques such as temperature-programmed desorption (TPD), reflection absorption infrared spectroscopy (RAIRS) and X-ray photoelectron spectroscopy (XPS). This enables the nature of the initial surface species to be identified, and their thermal stability to be measured in some detail. The effect of rubbing these adsorbate-covered surfaces is investigated, also in UHV, by measuring the friction coefficient and contact resistance during rubbing. The chemical composition of the wear track is followed using high-spatial-resolution Auger spectroscopy to follow the fate of the surface species caused by rubbing and to test the above postulate.

In addition, the effect of low-coordination sites formed on the surface by rubbing on the reactivity of gas phase lubricants is explored and finally, this strategy is used to understand the tribochemistry of borate esters on copper under mild rubbing conditions.